1. Field
The present invention relates generally to data communication, and more specifically to delay lock loops (DLL) for use in wireless communication systems.
2. Background
In a wireless communication system, an RF modulated signal from a transmitter may reach a receiver via a number of propagation paths (e.g., a line-of-sight path and/or reflected paths). The characteristics of the propagation paths typically vary over time due to a number of factors such as fading and multipath. To provide diversity against deleterious path effects and improve performance, multiple antennas may be used to receive the transmitted signal. If the propagation paths between the transmit antenna and the multiple receive antennas are linearly independent to at least an extent, then the likelihood of correctly receiving a data transmission increases with the use of additional receive antennas.
In a multipath environment, the signal received on each antenna may include a number of instances of the transmitted signal. If multiple receive antennas are used to provide receive diversity, then each propagation path results in a signal instance appearing at each of the multiple receive antennas. If these antennas are located within close proximity to one another, then the multiple signal instances received on these antennas for any given propagation path will be closely aligned in time but may have different (and possibly wide ranging) amplitudes and phases depending on the specific channel gains for the receive antennas.
At the receiver, each received signal is conditioned and digitized to provide a respective stream of data samples. A rake receiver may then be used to process one or more signal instances in each received signal. Improved performance may be achieved if the various signal instances in the multiple received signals can be processed and combined in a manner such that more of the signal energy is collected for a given data transmission.
One key challenge to achieving high performance is to properly sample each signal instance at the specific time instances associated with the highest signal-to-noise-and-interference ratio (SINR). If receive diversity is not used (i.e., only one receive antenna is employed), then a delay lock loop may be used to adjust the sample timing for each signal instance such that the received signal is effectively sampled (or resampled) at or near the optimum time instances. However, when receive diversity is used, certain challenges are encountered in deriving the proper sample timing for the multiple signal instances received on multiple antennas for each propagation path.
Moreover, additional challenges are encountered in deriving the proper sample timing for certain operating conditions. For a communication system designed to operate at low SINRs, such as an IS-95 CDMA system, the SINR of a signal instance is not quite as sensitive to errors or jitter in the sample timing. However, for a system designed to operate at high SINRs, such as an IS-856 CDMA system, errors or jitter in the sample timing may result in more noticeable degradation in the signal instance's SINR, which may then degrade performance.
There is, therefore, a need in the art for techniques to provide proper sample timing for receive diversity and/or high SINR operating environments.